The invention concerns an optical pick-up whereby a laser beam generated by a laser with a monitor diode is focused on an optical recorded medium by means of a focusing circuit, a periodic signal is supplied to one input terminal of a summation amplifier and to one input terminal of a multiplier, the output terminal of the multiplier is connected to another input terminal of the summation amplifier, the output terminal of the summation amplifier is connected to the positioning mechanism of the focusing circuit, and the actual value of the focusing circuit is supplied to still another input terminal of the summation amplifier.
The design and function of an optical pick-up for a compact-disk player are described in Electronic Components and Applications 6 (1984), 4, 209-14.
A focusing circuit for recording and playback equipment whereby data can be read from an optical recorded medium by means of a READ laser and recorded onto the medium by a WRITE laser is known from European Application 0 044 072.
The output terminal of a summation amplifier, to one input terminal of which a periodic signal is supplied from an oscillator, is connected in this focusing circuit to the positioning mechanism of the focusing circuit, usually a coil that displaces an objective lens. The periodic signal from the oscillator is also supplied to one input terminal of a multiplier, the output terminal of which is connected by way of a deep-pass filter to the second input terminal of the summation amplifier. The output signal from the focus detector, which is the actual value of the focusing circuit and is often called the focusing error, is supplied to the third input terminal of the summation amplifier, whereas the data signal detected by the laser beam and often called the high-frequency signal, is supplied to the second input terminal of the multiplier by way of a duty-cycle detector in order to establish the scanning situation. The focusing circuit focuses the laser beam very precisely on the optical recorded medium.
To ensure precise focusing and tracking of the laser beam in an optical pick-up of this type, however, the laser and the focus detector must be very precisely adjusted during the manufacturing process. Procedures of this type are very expensive because they take a lot of time and demand particular care.